In the preparation technology of liquid crystal display panels (e.g., the preparation technology of an array substrate and a color filter substrate and a technology for assembling the array substrate and the color filter substrate to form a cell), as it is required to accurately place a substrate at an ideal position during the transportation, clamping and processing of the substrate, the position of the substrate needs to be aligned and corrected prior to placement so as to ensure the accuracy of a corresponding operation. The alignment accuracy of the substrate directly influences the production quality of liquid crystal displays, so the accuracy of an alignment system is highly required.
An alignment system generally recognizes the position and shape of an alignment mark provided on a substrate, and thus realizes accurate alignment of the substrate. At present, the recognition of an alignment mark is generally realized by an alignment mark recognition device. A camera is provided in the alignment mark recognition device, and the camera can capture the position and shape of the alignment mark on the substrate by shooting. Then, the position of the substrate is adjusted according to the position and shape of the alignment mark, so that the substrate 4 may be accurately aligned finally.
In an alignment mark recognition device in the prior art, to enable the camera to clearly capture the position and shape of the alignment mark on the substrate, a light source box is provided between the camera and the substrate. As shown in FIG. 1, light emitted form a side light source is reflected onto the alignment mark 41 of the substrate 4 by a glass plate 5 in the middle of the light source box 21, and then the camera located above the light source box 21 receives the light reflected by the alignment mark 41 and thus captures the position and shape of the alignment mark 41. In this method for irradiating the alignment mark 41 by reflecting light emitted from the side light source by the middle transparent glass plate 5, the luminance on the alignment mark 41 is small, and the contrast ratio of the alignment mark 41 captured by the camera is low. As a result, this is very likely to result in the inaccurate alignment of the substrate, and the quality of liquid crystal displays is thus influenced.